1. Field of the Invention
The present invention relates to a rotation sensor, such as a steering sensor for an automobile, which is capable of measuring the angle of rotation of a shaft and torque acting thereon.
2. Description of the Related Art
With the recent movement toward the conservation of global environment, researches have been devoted to reducing the weight of a vehicle body, as a measure to reduce the exhaust gas from the automobile. One of such measures for lightening the vehicle body is to replace a hydraulic power steering assembly, which is one of the causes of increased weight of the vehicle body, with an electric power steering assembly. However, in the case of using an electric power steering assembly, an assisting force for a steering shaft needs to be computed using a computer. Accordingly, the electric power steering assembly requires two sensors, i.e. a rotation angle sensor (steering angle sensor) for measuring the rotational angle (steering angle) of the steering shaft and a torque sensor for measuring the torque applied to rotate the steering shaft. Thus, the number of components increases, making it difficult to reduce the weight of the vehicle body as intended.
Such a rotation angle sensor or torque sensor conventionally includes a type having mechanism where a brush is disposed in sliding contact with a conductive ring arranged concentrically with the shaft, for detecting the rotation angle or torque of the shaft. The mechanism, however, has a disadvantage that metal powder scatters due to friction when the brush and the slip ring, both made of metal, are in siding contact with each other. The metal powder undergoes a mechanochemical reaction with organic gas in the air, causing the formation of an insulating film and consequent unexpected increase of electric resistance.
It is possible to avert such increase of electric resistance by increasing the contact pressure of the brush and thereby scraping away the insulating film. However, this shortens the life duration of the slip ring.